Check with FMT, Florida Marine Tank in Henderson NC, they were the original supplier for my '04 Seafarer and are still building tanks for Grady. I bought my auxiliary tank from them last winter, I'll be getting my new main tank from them this winter. Some guys have outsourced their replacement tanks from other places and they seem to be satisfied. I'll stick with original supplier, less chance for an issue.
You can get a quote from Atlantic Coastal Welding in Bayville NJ. They build custom and to spec tanks and may have the specs for yours on file. They will ship to you (it's expensive) but their work is top notch. Their web address is https://speedytanks.com/
I had to replace a tank on a previous boat(non-grady). Florida marine tank was the OEM. I gave them the boat and tank info and they sent out the tank. I had talked to a couple of other tank manufactures, after needing specific measurements and needing a non standard size tank, it was just easier and quicker to go through FMT.